1. Field of the Invention
This invention relates to an improvement on a color temperature information forming device and an image sensing apparatus.
2. Description of the Prior Art
In picking up the images of objects by means of an image sensing apparatus, such as an electronic camera, the image of a white object is sometimes reproduced in a colored state due to the spectral sensitivity characteristic of an illumination light source. To avoid this, there has been contrived an automatic follow-up type color balance adjusting device which is arranged to measure the color temperature of a light source and to automatically adjust the color balance in an image sensing apparatus on the basis of information on the measured color temperature.
FIG. 1 of the accompanying drawings shows, in a block diagram, an example of the conventional automatic follow-up type color balance adjusting device. The illustration includes an R sensor 2 and a B sensor 3 which are arranged to have the same red (R) and blue (B) spectral sensitivities as those of an image sensor 1. These color sensors 2 and 3, respectively detect the R and B components of a light obtained from an object to be photographed under a light source. Each of the output signals iR and iB produced from the sensors 2 and 3 varies in a very wide range. Therefore, these signals are logarithmically compressed by logarithmic compression circuits 4 and 5 into signals log iR and log iB. The signals log iR and log iB are converted by a differential amplifier 6 into a difference signal log (iR/iB) representing the ratio between the R and B components of the light coming from the object. A control voltage deducing circuit 7 receives this signal log (iR/iB) from the differential amplifier 6. The circuit 7 then judges the color temperature of the light source on the basis of the ratio between the R and B components and deduces thereby control voltages CR and CB. These control voltages CR and CB control the degree of amplification of amplifiers 8 and 9 which are arranged to amplify signals produced from the image sensor 1 representing the R and B components of the light, respectively. These signals are thus color balance adjusted and then supplied to a signal processing circuit 13. The circuit 13 modulates and multiplexes these signals into the form of a standard TV signal of, for example, the NTSC color system.
In the case of a flickering light source, such as a fluorescent lamp, the flickering of the light takes place at a frequency twice as high as the commercial power supply frequency. In addition, the intensity of one color component of the flickering light differs from that of another. Therefore, the magnitude of the output signal of the differential amplifier 6 also varies to a great extent at a frequency twice as high as the frequency of the commercial power supply. While the flickering does not make much difference for human eyes under a fluorescent lamp light source, the color sensors precisely detect the periodic changes in colors resulting from the flickering of the light source.
To solve the above-stated problem of flicker, video cameras are generally arranged to have an analog low-pass filter inserted in between the differential amplifier 6 and the control voltage deducing circuit 7 for preventing the adverse effect of the flicker. However, in the case of an apparatus such as an electronic camera which must accomplish color balance adjustment in a short period of time after a power supply is switched on, the time constant of the analog low-pass filter is excessively large and the use of it is not appropriate, because the signal component that must be removed is of a low frequency of about 100 Hz. In other words, an apparatus like an electronic camera is occasionally required to take a still picture immediately (in an extremely short period of time) after the power supply is switched on and before the output of the above-stated low-pass filter becomes stable. In such a case, therefore, photo-taking operation is completed before stabilization of the output of the analog low-pass filter after the power supply is switched on. As a result, a picture is obtained before the color balance is adequately adjusted. This has presented a problem.